The present invention relates to a method for producing images on radiation sensitive recording mediums. The present invention is particularly useful in producing halftone images of an original image, although the method may also be used for producing other types of images such as alphanumeric characters on radiation sensitive recording mediums.
Printing processes commonly used in the graphic arts industry, i.e., newspapers, books, magazines, etc., involve depositing a series of dots of ink on a paper whenever it is desired to print all or a portion of an image, and depositing no ink when the absence of an image is desired. With such processes, when pictures such as photographs or prints are to be printed, the continuous tones of the original image are transformed into halftone images which are typically produced by a large number of ink dots of various sizes. The size of the ink dots correspond to the shades or tones to be reproduced. When the largest dots, and the spaces on the paper between the dots, are made small compared with the visual acuity of the human eye, i.e., they are subliminal to the eye, the dots and paper fuse visually and create or simulate various shades of continuous tones.
In the printing of color images, a series of separate halftone images are typically produced, one for each different color. These separate halftone images are processed in a conventional manner to provide a corresponding series of printing plates having halftone dots thereon. The printing plates are then used to print each of the different colors, such as by an offset process, on a recording medium such as paper so that the different colored dots from the different plates are superimposed over or adjacent to one another on the recording medium. The composite as a result of this printing process thus produces an image which simulates the original image. Generally, in such conventional systems, the different halftone images have the dots thereof oriented at different screen angles to minimize undesirable moire effects.
In accordance with early prior art methods and apparatus for producing halftone images on a film, a light image of an original (such as by passing light through a photographic negative) is directed through a halftone screen element onto a film to thereby expose the film in a series of dots which simulate the light image of the original. The screen element consists of two series of parallel lines oriented at right angles, and produces a dot image in which the highlights of the image are represented by small dots and the shadows are represented by large dots. In other words, the size of the dots created in this manner vary according to the tones of the original image. The photographic film having the dot image thereon is then processed in a conventional manner and used to produce a printing plate which can then be used in an offset printing process.
More recently, halftone reproductions of images are produced electronically by using an exposing head having a number of light source elements arranged in a row, and by providing relative movement between the row of light sources and a light sensitive medium to be exposed, such as for example a photographic film. With such a system, the row of elements generates a line of light across the width of the light sensitive medium to be exposed, and the exposed area is then extended in a direction perpendicular to the line of light by relative movement between the medium and the line of light from the light source elements. The illumination of the light source elements during this relative movement is selectively controlled by electronic signals so that a series of halftone dots are created on the light sensitive medium. Typically, the electronic signals for controlling the illumination of the light source elements are made up of an electronically generated screen representing signal and a picture representing signal, the picture representing signal being common to all of the light source elements for the period in which each single dot is exposed. The screen representing signals are such as would be produced by scanning a vignetted contact screen at a resolution much finer than that used for picture scanning. In essence, with this technique, the continuous composition of the image to be reproduced is transformed to produce a plurality of dot character images on the light sensitive medium by building up each dot character from a line of light which is moved in a direction perpendicular to the direction of the line. The size of the dot character corresponds to the length of the line of light (and thus to the number of light source elements illuminated), and the length of time the light source elements are illuminated as the recording medium and light sources move relative to one another.
More particularly, in one typical type of system for electronic halftone image reproductions, the exposing head comprises six light source elements which may be selectively illuminated and which are arranged in a row to produce a line of light for exposing a light sensitive recording medium, such as for example a photographic film, which is placed on a cylindrical drum and adapted to rotate about an axis extending parallel to the row of light source elements. As the cylindrical drum is rotated, the light source elements are controlled so as to be illuminated intermittently to expose the light sensitive medium, the row of light source elements also being moved, such as for example by means of a lead screw arrangement, to traverse across the width of the light sensitive medium. Thus, the row of light source elements are moved along a line extending in a direction along the row, and the light sensitive medium is moved in a generally perpendicular direction therepast so that a series of helical sections are progressively exposed by the light source elements. By controlling the number of light source elements illuminated and the time of illumination during this relative movement, a series of halftone screen dots of varying sizes may be produced on the light sensitive medium. That is, each dot area is produced on the recording medium by illuminating one or more of the light source elements for a selected period of time, the width of the dot being determined by the number of light source elements illuminated and the length of the dot area being determined by the time the light source elements remain illuminated.
In many of these prior art systems for electronically producing halftone images, the light or radiation generated by each of the light source elements is substantially of the same intensity and the intensity is substantially constant during each interval. As such, the dot areas produced are what is normally termed "hard" or uniform density dots, i.e., dots produced with constant intensity radiation and having a small density gradient across the dot area. Generally, the intensity level of the radiation is much greater than that necessary to simply produce an image on the light sensitive medium.
U.S. Pat. No. 4,025,189 to Pugsley discloses a system in which "soft" or nonuniform density dots are produced, i.e., dots which are produced by radiation in which the intensity profile has sloping sides decreasing towards the edges. The dot areas produced in this manner have a density gradient across the dot area in which the density at the lateral edge of the dot area is lower than the density in the central portion of the dot area. As discussed in this reference, the reason or desirability behind producing such soft dots is to permit the length of the line of light generated by the light sources in the exposing head to be adjusted continuously so that a substantially continuous variation in dot size becomes possible.
More particularly, with typical electronically generated halftone image reproduction systems, the number of light source elements for producing the halftone dots on the medium is limited by the expense and complexity of the resulting optical system. Consequently, there is a practical limit on the number of gradations or steps of dot size obtainable with such systems. That is, when only six elements are utilized to produce a scanning line, with conventional systems in which the intensity profile of radiation produced by each light source is constant, the number of variations in size of the width of the dot elements created would be six, corresponding to the number of elements illuminated during the generation of the single dot area. In order to provide for greater variation in dot size without increasing the number of light source elements, Pugsley contemplated varying the illumination intensity of a light source element which is ajacent to an unilluminated light source element so that the rate of decrease of the light intensity with distance from the adjacent fully illuminated light source element could be adjusted. Thus, when there is a threshold of response in the reproduction process, such as when the recording medium to be exposed is a lithographic film having a response threshold, the progressive variation of illumination intensity of a light source element at the end of a segment of illuminated light source elements progressively adjusts the length of the line which will be printed at the portion of the required dot area. In other words, with the Pugsley arrangement, the length of the illuminated line of light which is directed onto the recording medium as the recording medium moves generally perpendicular past the light source elements may be adjusted continuously instead of incrementally, and thus the width of the dot being generated may be continuously varied. It should also be appreciated that with the Pugsley arrangement, the length or height of the dot created is still controlled by the length of time that the light source elements remain illuminated.
Another problem of the prior art arrangements for electrically generated halftone image reproductions relates to halftone image reproductions at different screen angles, such as for example with respect to color reproductions. As is well known in the art, color image reproductions are created by utilizing a number of different halftone image plates, each plate representing a dot image of a different color. To reproduce a color image, a printing operation, one for each different color, is performed with the halftone plates so that the halftone dots thereof are superimposed over or adjacent to one another. In such systems, each halftone image reproduction will have its dot centers arranged in rows and columns with the overall rows and columns being oriented at the different screen angles. For instance, typically, the halftone reproduction for one color is produced at a zero degree screen angle with the remaining reproductions being produced at different screen orientations, e.g., +15.degree., +45.degree. and -15.degree. for conventional four-color process reproductions.
As can be appreciated, it is desirable that the different halftone reproductions for different screen angles each be produced with the same apparatus, for example, the exposing head and rotary drum arrangement of the prior art systems described hereinabove. While such conventional arrangements in which the line of light source elements or signals are moved transversely across a screen which is being rotated or moved in a perpendicular direction relative to the line of signals may produce accurate and precisely controlled dot patterns for zero screen angles, the dot patterns produced for angled screen arrangements are not precise. This results from the fact that the exposed segments comprising each dot area actually are segmented bands having a width corresponding to the width of the light source elements which are illuminated for a certain period of time and a length or height corresponding to the time that the light source elements remain illuminated. This is the case whether zero angle reproductions are being generated or angled reproductions are being generated since the dot area is still being generated by moving the recording medium transversely to the row of light source elements and illuminating the light source elements for periods corresponding to the desired size of the dot to be produced. Thus, it will be appreciated that precisely controlled shapes for the dot areas can not practicably be produced with such prior art arrangements, and occasionally the reproductions are objectionable.
A still further problem with conventional electronically generated halftone reproductions relates to the ability to accomplish color corrections. Generally, color correction must be accomplished electronically by adjusting the picture representing signals generated by scanning of the original image. While equipment has been developed to accomplish this, acceptable correction of the color values have not always been produced and it is thus necessary to generate additional halftone reproductions having different color correction values.
Here, it should be noted that with the prior art systems in which the light image of an original is directed through a screen element, color correction may typically be accomplished by an etching process in which the size of the dot areas is adjusted after creation of the dot image. Such etching processes conventionally involve applying an etching solution to selected dot areas of the reproduction image which serve to alter the size of the selected dot areas, generally by reducing the size of the selected dot areas. To be able to utilize such an etching process, it is generally necessary that a significant photographic density gradient be provided across the dot area so that the etching solution will only attack the low density portions of the dot area. Such density gradients are inherently produced with halftone image reproductions created by directing a light image through a screen element. However, with electronically generated halftone reproductions, there generally is not a significant density gradient across the dot area; rather, only "hard" type dots are produced. Thus, it has not previously been possible to utilize an etching process for effective color correction with electronically generated halftone reproductions.
These and other disadvantages of the prior art arrangements are overcome with the method in accordance with the present invention.